Cannot keyframe visibility in outliner "Groups" view
The group visibility in the outliner is just there for convenience to control the visibility of objects, but is not actually a property of the group.

Objects can be apart of more then one group so it wouldn't really make sense for visibility to be a group property.

Some alternatives are...


Assign the same action to all objects, then use that action to control visibility.May conflict with existing animation, so you could check on using the NLA too.
Assign a driver to objects visibility and use a single toggle to control it.
Use a frame change callback from Python and have a script inspect some property and set the visibility of the group based on it.Basically a hack to do something similar to a driver
Instance the group and change the visibility of that.Realize this isn't always a workable solution.
Solve it elsewhere, use compositing for example to enable/disable different scenes, in some cases material alpha can be used too.